There could be life on the planet Venus, US scientists have concluded in a report in the journal Astrobiology.

The existence of life on the planet's oven-hot surface is unimaginable.

But microbes could survive and reproduce, experts say, floating in the thick, cloudy atmosphere, protected by a sunscreen of sulphur compounds.

Scientists have even submitted a proposal for a Nasa space mission to sample the clouds and attempt to return any presumed Venusians to Earth.

One lifeform's deadly radiation may be another lifeform's lunch

David Grinspoon, South West Research Institute in Colorado

"Venus is really a hellish place," said Professor Andrew Ingersoll, of the California Institute of Technology.

"If you could get through the sulphuric acid clouds down to the surface of Venus you'd find it was hotter than an oven. You could melt lead at the surface of Venus and there'd be no water."

But it was not always like that. Earth and Venus are in many ways sister planets.

"Current theories suggest that Venus and the Earth may have started out alike. There might have been a lot of water on Venus and there might have been a lot of carbon dioxide on Earth," Professor Ingersoll explained.

But all that was to change. On Earth, life in the oceans took in carbon dioxide and turned it into limestone. On Venus, 30% closer to the Sun, any oceans boiled away and the water vapour added to the runaway greenhouse effect.

Venus became our planet's ugly sister. Its make-over, which occurred billions of years ago, has left a surface where the pressure is crushing.

Arrested development

But, according to Louis Irwin of the University of Texas at El Paso, the changes on Venus may have been slow. "It may well have been Earth-like long enough for life to either emerge or be transported there," he said.

Once established, life would have adapted to every environment, just as it did on Earth.

Two years ago, Austrian scientists discovered bacteria living and reproducing within clouds on Earth. The same could have been true on Venus. Then, as the surface became hot and dry, the clouds might have become life's only refuge.

Venus may once have been Earth-like but now surface temperatures can melt lead

The Venusian clouds are high in the atmosphere, where the temperature and pressure are quite Earth-like. There is even water present, though it is in the form of concentrated sulphuric acid.

But we now know of organisms that thrive in very acidic environments on Earth.

"If you think about what life needs in a broad sense then the clouds of Venus might actually be a habitat where something could live," explained David Grinspoon, of the South West Research Institute in Colorado.

Another problem could be UV radiation from the Sun. But Dirk Shulze-Makuch, also at El Paso, thinks Venusian bacteria could make use of a natural chemical sunscreen there.

"When we looked at the composition of the atmosphere, we thought that sulphur compounds are actually an ideal sun block for microbes."

David Grinspoon speculates that the organisms might even have evolved ways of making use of the UV, much like Earth plants use visible light for photosynthesis.

"One lifeform's deadly radiation may be another lifeform's lunch," he added.

But will we ever know if there is truth behind the speculation? Louis Irwin and his colleagues have a proposal in with the US space agency, Nasa.

"We would send a probe to Venus that would drop probably a collector tethered to a balloon-like floating spacecraft, it would collect samples of the cloud droplets and then blast off from the Venusian atmosphere for return eventually to Earth."

And what are the chances of finding live Venusians? David Grinspoon is in no doubt: "If they're there, I think we will find them eventually."

Venus Across The Sun, a look ahead to 8 June's Venus transit, is broadcast on BBC Radio 4 (2100BST) and the BBC World Service (1005BST) on Wednesday, 26 May.